In a conventional type of anti-collision system for a vehicle, a radar system which utilizes electromagnetic waves or light is used for detecting the distance between an obstacle ahead and the vehicle and the relative velocity between thereof. Signals which are respectively indicative of the distance and the relative velocity are utilized for deciding whether there is a possibility of collision or not through the consideration of the vehicle speed and other conditions such as the condition of the road surface. When the anti-collision radar system detects a possibility of a collision, the system generates a collision imminence signal with which an alarm signal is issued and/or the brakes of the vehicle are automatically actuated.
When such an anti-collision radar system is utilized, the frequency of the generation of the collision imminence signals is very low if the driver of the vehicle operates the vehicle correctly. According to experiments the average number of times of generation of such collision imminence signals is less than two times during 20 kilometers travel of a vehicle where the number obtained by experiments includes a number of collision imminence signals which are produced by miss detection, i.e. detection of obstacles such as poles standing along the road as dangerous obstacles. However, when the vehicle driver's condition is unusual, for instance the driver is very tired, sleepy or drunk, the driver cannot operate the vehicle correctly in a safe manner. Therefore, the frequency of the generation of the collision imminence signal is apt to become high since the vehicle tends to move meanderingly and/or the distance between an obstacle ahead such as a vehicle ahead and the vehicle is dangerously short.
Since such undesirable conditions of the vehicle driver are not pertinent for normal and safe driving of a vehicle, the driver should stop the vehicle as soon as possible in order to prevent a dangerous collision. However, in a conventional type of anti-collision system there is no means for checking if the driver's operation of the vehicle is good enough and thus the driver must decide whether his operation of the vehicle is normal or not by himself.